SIZE, FORM, AND LOCATION OF THE HEART
The adult heart is shaped like a blunt cone and is approximately the size of a closed fist. It is larger in physically active adults than in less active but otherwise healthy adults. The heart generally decreases in size after approximately age 65, especially in people who are not physically active. The blunt, rounded point of the heart is the apex and the larger, flat part at the opposite end of the heart is thebase.
The heart is located in the thoracic cavity between the two pleural cavities that surround the lungs. The heart, trachea, esophagus, and associated structures form a midline partition, the mediastinum (me′dē-as-tı̄′nŭm; see figure 1.13). The heart is sur-rounded by its own cavity, the pericardial cavity (peri, around + cardio, heart) .
It is important for health professionals to know the location and shape of the heart in the thoracic cavity. This knowledge enables them to accurately place a stethoscope to hear the heart sounds, to place chest leads for an electrocardiogram (described later), or to administer cardiopulmonary resuscitation(kar′ d̄e -̄o -p̆u l′mo-n̄a r-̄e r̄e -s̆u s′ i-t̄a -shun; CPR). CPR is an emer-gency procedure that maintains blood flow in the body if a person’s heart stops (see A Case In Point, “Hands Only Cardiopulmonary Resuscitation (CPR),” later).
The heart lies obliquely in the mediastinum, with its base directed posteriorly and slightly superiorly and its apex directed anteriorly and slightly inferiorly. The apex is also directed to the left, so that approximately two-thirds of the heart’s mass lies to the left of the midline of the sternum (figure 12.3a). The base of the heart is located deep to the sternum and extends to the level of the second intercostal space. The apex is deep to the left fifth inter-costal space, approximately 7–9 centimeters (cm) to the left of the sternum near the midclavicular line, which is a perpendicular line that extends down from the middle of the clavicle (figure 12.3b).
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